CHALLENGES IN DESIGNING ONCE-THROUGH CIRCULATING WATER SYSTEMS. Santosh Pansare Black & Veatch Consulting Private Ltd. India

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1 CHALLENGES IN DESIGNING ONCE-THROUGH CIRCULATING WATER SYSTEMS Santosh Pansare Black & Veatch Consulting Private Ltd. India Once-through circulating water systems have been the most popular arrangement for power plant cycle heat rejection. In this system, water is taken from a river, lake or ocean, pumped through the plant condenser and discharged back to the source. It has the most efficient cycle heat rejection system design with the lowest capital and operating costs; however, a lot of planning, detailing and collaboration are required early in the project to successfully optimize the system design. Once-through systems are generally more restrictive because of varying pump suction conditions. Intake and outfall require thermal plume model studies to determine appropriate locations and arrangements to control temperatures at intake, and to mitigate possible impact on marine life. An environmental permit is required and careful techno-economic evaluation of multiple intake and outfall options must be carried out. A sump model study and transient analysis of the intake structure are vital in ensuring that the circulating water pump intake structure is sufficiently sized and equipped to mitigate the undesirable flow conditions for circulating water pumps in all possible operating scenarios. Appropriate siphon recovery can be considered by selecting an optimum seal weir elevation to reduce the circulating water pump head. Hydraulic gradient calculations are required to understand vacuum levels formed in the system. Vacuum priming systems are 1

2 required to establish system prime during startup and operation. A seal weir constructed with removable sections can help in commissioning to keep the pump on its curve. Material of construction for various equipments such as screens and stop logs, circulating water pumps, pipes and valves shall be compatible with the fluid being handled. Each of these factors must be considered and analyzed for specific site conditions. This paper provides an overview of the challenges associated with once-through system designs and also shares some of the recent experiences in the design of a once-through system using seawater. INTRODUCTION Condenser cooling water is supplied by the circulating water system. It takes the heat from the condenser and rejects it to a heat sink such as a cooling tower or a body of water. In a once-through system, water taken from a body of water such as a river, lake or ocean is pumped through the condenser and returned back to the source. The heat is then slowly transferred to the atmosphere by evaporation, convection and radiation. In a closed-cycle or recirculating cooling system, the circulating water serves as an intermediate heat transfer medium from which the waste heat is directly rejected to the atmosphere. The main disadvantages of recirculating cooling systems are lower plant efficiencies, higher capital costs and higher maintenance compared to once-through cooling systems. A schematic of a once-through system is shown on Figure 1. The major equipment in a once-through system includes the circulating water pumps, circulating water piping and valves, condenser, closed-cycle cooling water heat exchangers and/or auxiliary cooling water heat exchangers, vacuum priming system, traveling water screens, intake structure and outfall structure. Debris filters and condenser tube cleaning systems may also be used, depending on the client s preference. For many years, the once-through system has been the most popular arrangement for power plant cycle heat rejection systems. A once-through system has two significant advantages over a closed-cycle system. First, the relatively low temperature of most water sources used for once-through cooling makes this the most efficient cycle heat rejection system design. Second, the simple system arrangement typically makes once-through cooling the cycle heat rejection system design with the lowest capital and operating costs. 2

3 Figure 1. Once-Through System The main disadvantage of a once-through system is that the heated water is discharged back to the original water source, where the added heat is gradually dissipated to the Earth s atmosphere. However, it may take a long time for the source water temperature to return to normal, or it may reach a new equilibrium temperature higher than the normal temperature for the life of the plant. There are other disadvantages to the once-through system. The use of a once-through cooling system is greatly affected by local water quality regulations. Also, the large cooling water requirement of a once-through system limits potential plant sites to locations near large rivers, lakes and oceans. Although a once-through system has the most efficient cycle heat rejection system design with the lowest capital and operating costs, a lot of planning and collaboration is required on a number of different aspects of a once-through system early in the project. The following sections briefly discuss the design considerations of various components of a once-through system, and also share some of the recent experiences in designing a seawater based oncethrough system. SEA INTAKE AND OUTFALL STRUCTURE For a once-through system, intake structures are located on a river, lake, ocean or cooling pond. The primary function of an intake structure is to have environmentally acceptable withdrawal conditions from the water source. An outfall structure is typically included as a means to return the discharge circulating water flow to the waterway. Many once-through 3

4 system piping arrangements can incorporate siphon recovery to reduce the system pumping head requirement. This is discussed separately in the following sections. Environmental permits typically require determination of flow and temperature patterns around the circulating water discharge to estimate the thermal plume impact on the body of water. A computational fluid dynamics (CFD) model is prepared early in the project to determine the impact of circulating water discharge on the body of water. The requirements include meeting the expected inflow and outflow with the given temperature rise across the condenser. Most countries have set restrictions on allowable temperature rise from intake to outfall (typically 5 to 6 degrees C). Apart from that, specific restrictions are imposed on the temperature rise within the body of water to protect marine life. A typical example of this would be not allowing temperature over coral to reach beyond a certain level. A CFD model helps determine the thermal plume and helps predict the temperature rise in vertical and horizontal directions. A typical result of CFD thermal plume model is shown on Figure 2. Recirculation between outfall and intake is prevented by ensuring that the intake always draws from cooler water below the surface warmed water field. The velocities at the intake head are controlled to prevent drawdown of warmed water from the surface and inflow of debris and solid material from the seabed or from the water column. The outfall structure is typically provided with diffusers to impart an optimal discharge direction and velocity to the discharge circulating water for proper dispersion into the body of water. Figure 2. Thermal Plume CFD Model 4

5 Multiple options are available for the intake and outfall from the body of water depending on the location and arrangement. The primary options include channel-type surface intake and outfall and pipe-type seabed intake and outfall. The seabed is primarily categorized as sandy or rocky bottom. In sandy bottoms, there is a tendency for the movement of sand into the intake channels, causing possible erosion of channel walls or resulting in sand being deposited in the channel. This deposit of sand could result in blockage of the channel, requiring redredging to keep the channel open. On the other hand, a rocky bottom poses challenges in laying pipe-type systems. Pipes laid in an uneven bed will be supported only at high spots, resulting in high localized stresses. In addition, the rocky bottoms increase the danger of abrasion to the pipelines. Generally, a sandy sea bottom calls for a pipe-type intake system, whereas a rocky bottom calls for a channel-type intake system. The recent project being discussed uses pipe-type intake and outfall design that draws and returns seawater at seabed elevation. Pipe material options that are composite steel and concrete pipes and high density polyethylene (HDPE) were considered and HDPE was selected based on the techno-economic evaluation. The HDPE pipes are supported on seabeds using concrete saddles. The intake heads that are mounted on top of intake pipes draw water from the seabed horizontally. Refer to Figure 3 for the typical intake head structure. Screens are provided on intake heads to protect the intake pipeline from marine life. Chlorination dosing pipes are routed to the intake structure to perform shock dosing to control the biofouling. The intake and outfall pipes are provided with expansion joints at an onshore interface to ensure required flexibility. Suitable cathodic protection is provided wherever steel material components are used. The outfall pipes are provided with diffusers to direct the water in a seaward direction. CIRCULATING WATER PUMP INTAKE STRUCTURE The plan view of a typical intake structure for vertical wet pit pumps is shown on Figure 4. Two major design parameters are considered for the once-through intake structure. The first parameter is the hydraulic/hydrologic characteristics of the water source. For a river or lake intake structure, the water level may vary on a seasonal basis. For an ocean intake structure, the water level varies hourly depending on tidal influences. In either case, the intake structure must be designed to provide adequate pump submergence and net positive suction head (NPSH) at low water levels, while ensuring that the pumps and motors will not be flooded at high water levels. 5

6 Figure 3. Intake Head Structure with Concrete Block Support Figure 4. Plan View of Circulating Water Pump Intake Structure The second design parameter is the environmental regulations applicable to each plant location. An intake structure located on a natural body of water is usually subject to regulations concerning the water approach velocity to protective intake screens, intake structure location and other site-specific environmental concerns. Intake structures located on a lake, river, ocean or cooling pond require a screening system for debris removal. To be considered practical for use in protected power plant circulating water pumps, a screening system must effectively screen the required amount of water without clogging or allowing bypass of screened materials. It is important that the 6

7 screening system is maintainable without interfering with the requirements of the cooling water supply. Some countries have specific environmental regulations that require the screening system to be effective in screening and protecting aquatic life. Screening systems that satisfy the above requirements are typically classified into traveling screen systems and passive screen systems. The most commonly used traveling screen is the vertically rotating, single-entry, single-exit, through-flow screen mounted facing the water source. The vertical traveling screen consists of screens attached to a continuous belt that travels in the vertical plane between two sprockets. The screen, which is typically 10 mm mesh, is usually supplied in individual removable panels referred to as baskets or trays. The entire screen assembly is supported by two or four steel posts. The drive for the screen system is typically a two-speed drive, with normal operation at low speed to reduce equipment wear, and at highspeed operation during periods of high debris loading. Screens are available in vertical lengths up to 30 meters and widths up to 4 meters. Operation of the system can be performed manually at regular intervals or automatically through continuously monitoring the differential pressure drop across the screen. A typical differential head to initiate operation of the screens is 150 to 250 mm of water. Another type of vertical traveling screen used in power plant intake facilities is the dualflow screen. Dual-flow screens are installed with the screens in parallel to the direction of water flow to the intake structure. The advantage of the dual-flow screen is that twice as much screen area per screen is available compared to the through-flow arrangement. There are several disadvantages associated with the vertical traveling screens. Experience has shown that the screens require high maintenance, especially if located in a high debris or sediment-laden environment, or if located in a severely corrosive environment such as seawater. In through-flow arrangements there is sometimes debris carryover to the pump side of the screen. Also, the screens may be environmentally unacceptable because of damage to aquatic life. A fish handling and bypass system is available to attempt to save fish impinged against the screens. The major difference between traveling screens and passive screens is that passive screens have no moving parts. Passive screens are typified by low approach velocities, low through-screen velocities and minimum debris impingement and blockage. Any debris or material that becomes impinged on the screens can be removed by a periodic air or water 7

8 backwash. Alternatively, scrappers can also be provided which can clean the passive screens periodically. The screens typically are designed for a maximum intake velocity of 0.15 meters per second through the screen. The low velocity reduces the flow forces that cause the entrainment and impingement of debris and aquatic life. The intake structure must provide acceptable pump suction hydraulic operating conditions at all possible water levels, accommodate the selected screening system and satisfy applicable environmental regulations. The commonly used standard for intake structure sizing is the Hydraulic Institute Standards. The basic criteria considered for the design of the intake structure is to provide uniform flow approaching the pumps across the width of the pump cell by dissipating the turbulence well in advance of the pumps. All flow obstructions should be streamlined to minimize flow separation near the intake structure. Average velocities must be kept below 0.6 m/s on the approach to the pump sump and 0.3 m/s or below on the approach to the pump bell mouth. In addition, the trash racks and screens should be located so they can also act as flow straighteners. The recent project being discussed uses dual-flow traveling screens coupled with passive screens. The screen material is carbon steel with cathodic protection and coating. The passive screens are provided with a common scrapper to clear them regularly. The traveling screens are provided with screen wash pump connections for regular wash down. A small transient analysis is performed for the intake structure to determine the volume sufficiency in transient events such as all circulating water pump trips. The transient analysis was performed to check the following two possibilities: When all the running circulating water pumps trip and the seawater intake momentum continues to draw in water, what is the increase in water level and are there chances of overflow circulating water pump intake structure? When one or more circulating water pumps are started with no momentum in place for seawater, what is the drop in water level and are there any chances that water level will drop to pump trip level? Through this transient analysis, it was concluded early in the project that the selected size of the circulating water pump intake structure is sufficient. 8

9 INTAKE STRUCTURE MODEL STUDY Sump model testing is recommended by the Hydraulic Institute for intake structures using pumps with capacities greater than 4,700 liters per second and for smaller pumps if there are conditions in the intake approach that generate unusual circulation. Intake structure modeling offers several benefits including the following: Possibly reduces costs by optimizing the size of the pump cells. Identifies the need for intake structure flow baffles, fillets and other pump structure modifications required to eliminate pump damaging surface and subsurface vortex formations. Provides physical verification of the effectiveness of these modifications. Helps reduce maintenance and improve efficiency for the pumps. These tests, as determined by the designer and pump manufacturer, would consist of a number of simulated flow conditions within the circulating water pump intake structure to confirm the intake structure design. Normally, these tests are included in the pump purchase specifications and sometimes become part of a combined pump and intake structure arrangement guarantee. These simple, inexpensive tests could preclude limited pump operation, pump damage and costly redesign of the intake structure after installation of the facility. There are many good references for pump intake piping and structure design that a designer should consult during pump selection and detailed design. The Hydraulic Institute offers many good recommendations, and the specific pump manufacturers should also be consulted for their recommendations. The sump model study was performed on the recent project being discussed here. The outcome of this model was the requirement of using some flow straightening walls to avoid disturbances near circulating water pumps. CIRCULATING WATER PUMP OUTFALL STRUCTURE Once-through systems typically use a siphon piping arrangement. It works on the principle that no pumping head is lost in a pumping system between two points having the same elevation because of elevation differences that may occur between the points. When pumping from one point to another at the same elevation, the only losses are caused by friction and valves, 9

10 elbows, etc. Most once-through systems can use a siphon to some degree because the condenser elevation is usually well above the water level of the water source. The siphon principle holds true provided that the circulating water piping flows full and is free of vapor and air. These requirements impose a limiting height for an effective siphon. The pressure in a siphon is a minimum at the highest point in the system, typically the top of the outlet condenser water box. To prevent vaporization of the liquid at the highest point, the pressure must exceed the vapor pressure of the water. Thus, the siphon may need to be broken in the circulating water discharge piping ahead of the outfall structure by installation of a seal well. The seal well exposes the circulating water flow to atmospheric pressure. The elevation of the seal well is determined so as to maintain sufficient back-pressure in the condenser outlet water box to prevent flashing of the circulating water. A minimum 1,800 mm of water absolute pressure head at any point in the system is recommended. The elevation of the water in the seal well is typically maintained by a sharp-crested weir formed with adjustable stop logs. Stop logtype construction of seal weir is recommended because it gives flexibility to adjust the weir height to keep the pump on its curve if the calculations do not accurately predict reality. In a seawater system, the back-pressure from the sea at high astronomical tide (HAT) level also imposes restrictions on seal weir height. While finalizing seal well elevation, sufficient (300 mm to 500 mm) free fall margin should be maintained to ensure free discharge over the seal well with sea in HAT condition. CIRCULATING WATER PUMPS Circulating water pumps are high-capacity low-head pumps that provide the cooling water flow for the circulating water system. Because of their large size and continuous operation, circulating water pumps must be carefully selected for economical and reliable operation over the lifetime of the plant. Circulating water pumps are typically selected from one of three pump designs: vertical wet pit, horizontal dry pit and vertical dry pit pumps. For oncethrough systems, vertical wet pit pumps are most commonly used. Vertical wet pit pumps are typically of the mixed flow, single-stage, single-suction type for circulating water service. Location of the motor directly above the pump column minimizes horizontal space requirements. Vertical wet pit pumps may be of pull-out or nonpull-out design. Pull-out design allows the rotating elements and critical nonrotating components such as the 10

11 impeller shroud and pump bowl/diffuser/volute to be quickly removed without removing the column or disconnecting the pump discharge. Nonpull-out design has a 20 percent to 25 percent lower capital cost; however, pump disassembly is more difficult and requires a longer pump outage. Another design variable for vertical wet pit pumps is the location of the discharge relative to the baseplate. An above-floor discharge indicates that the pump discharge is above the baseplate, whereas a below-floor or belowground discharge refers to the opposite. The below-floor discharge is more difficult to disconnect because access to the discharge is usually limited. Because disconnecting the discharge is required for disassembly of nonpull-out pumps, below-floor discharge combined with nonpull-out design may create maintainability problems. Other pump types that are horizontal and vertical dry pit pumps are not discussed here. The selection of a circulating water pump for a specific circulating water system application requires an evaluation of several design criteria. Pump design criteria include pump capacity and total developed head, NPSH, submergence, suction specific speed and rotative speed. Design capacity per pump is determined based on the design circulating water flow rate, including main condenser flow and auxiliary cooling water flow, and the number of pumps. The design circulating water flow rate is determined based on the maximum circulating water requirement and the number of pumps being provided. Typically, 2x50 precent configuration is preferred for circulating water pumps. The Hydraulic Institute Standards require circulating water pump manufacturers to meet the design capacity with margins of plus 10 percent and minus 0 percent. For this reason, no flow margin is included in the pump design capacity. No head margin is included in the design total dynamic head (TDH) due to conservatism in the head calculation. Also, the Hydraulic Institute Standards require manufacturers of low-head pumps to meet the design TDH with margins of plus 5 percent and minus 0 percent. For circulating water pumps, the basis for the net positive suction head required (NPSHR) value is dependent on the pump operating scenario or runout conditions. Satisfying NPSH criteria at the pump design point and appropriate runout points generally ensures that NPSH requirements will be met at all points of operation. However, the entire range of pump operation must be checked for NPSH acceptability. 11

12 Submergence is a measure of the water depth in the pump sump above the pump suction. Submergence must be adequate to meet NPSH requirements and to prevent vortexing. The Hydraulic Institute has provided guidance on minimum submergence to prevent vortexing for vertical wet pit pumps. These recommendations, along with recommendations provided by the pump manufacturer, should be considered when designing the pump intake structure for vertical wet pit pumps. Submergence may also be set by the sump depth required to meet traveling screen or trash rack approach velocities. The minimum acceptable submergence is established based on meeting all of the above criteria. In the selection of a pump for a once-through system, the variance in the level of the cooling water body dictates the required depth of the pump suction to satisfy submergence requirements at the low water level. The required depth of the pump suction may make use of a dry pit pump impractical because of the required depth and complexity of the intake structure. Also, a deep pump suction may require a very long vertical pump column, and may lead to consideration of a short column vertical pump. Equally important, the possibility of flooding must be considered if a dry pit or short column vertical pump is selected, in which case the pump drives cannot be mounted above flood level. Each of these factors must be weighed and analyzed for the specific site conditions. On the project in consideration, it was found that the nonpull-out design with aboveground pump discharge is at least a 20 percent to 25 percent less expensive option compared to a pull-out design with belowground pump discharge. The material of circulating water pump components varies based on the water quality. For seawater application, the requirement of super duplex or duplex stainless steel can be avoided by using impressed current cathodic protection except for complex geometry-type shapes that are shaft, shaft sleeves, keys, etc. CIRCULATING WATER PIPING AND VALVES The line sizing of circulating water piping should be determined by velocity. However, if the pipeline is significantly longer than normal, e.g., a once-through system with long separation between the intake and discharge, the line sizing may be determined by other criteria, such as pressure drop or water hammer mitigation. For pressure drop calculation, the absolute roughness 12

13 of pipe internal surfaces should include an allowance of 15 to 20 percent over the new and clean absolute roughness. Generally, most of the circulating water piping is routed underground. However, branch lines that are routed to the closed-cycle cooling water heat exchangers and auxiliary cooling water heat exchangers are routed aboveground. The aboveground piping shall be routed as directly as possible, and local high spots shall be avoided. The purpose of this routing is to mitigate water hammer potential. A hydraulic gradient calculation shall be performed, as discussed below to determine the potential for vapor cavity formation at high points. Depending on the type of water, the material of circulating water pipe and its coating and cathodic protection requirements change. For freshwater applications, carbon steel or fiberglass reinforced plastic (FRP) pipes are the preferred aboveground piping materials. For seawater aboveground application, rubber lined carbon steel and FRP are the options for smaller size pipes (less than 500 mm), whereas larger sized pipes can use carbon steel pipes with impressed current cathodic protection and coal tar epoxy interior coating. For underground piping, prestressed concrete pipe is a preferred option over carbon steel pipes with cathodic protection. Smaller underground pipes (less than 500 mm) could use HDPE material. All the circulating water piping and inline components including condenser water boxes in once-through systems are typically designed for full vacuum, apart from the positive design pressure that is calculated based on circulating water pump shutoff head. Pitot tubes are used for the measurement of circulating water pump flow to the condenser. The purpose of circulating water valves is to isolate equipment and to control the flow of water in the system. The pump discharge valves shall have motor operators. CONDENSER Condensers are classified as single pressure or multipressure, depending on whether the circulating water flow path creates one or more turbine back pressures. The condenser is described further as either a single-pass or a two-pass type, depending on the number of parallel water flow paths through each shell. The portions of the condenser that come into contact with the circulating water system include the condenser water box tubes and tubesheets. The materials of construction for water boxes shall be carbon steel with suitable coating. 13

14 Type 304 stainless steel is typically used for condenser tubes for freshwater systems with chloride concentrations less than 250 parts per million (ppm); for chloride concentrations greater than 250 ppm Type 316 stainless steel is recommended. For systems with chloride concentrations greater than 1,000 ppm, the condenser tube material should be titanium. Copper bearing tube materials shall not be used because of condensate water chemistry issues. The tubesheet material shall be compatible with the condenser tube material with regard to galvanic corrosion. Depending on the client s preference, automatic debris filtration systems and/or condenser online tube cleaning systems are used to prevent macrofouling of the condenser tubesheets and condenser tubes. AUXILIARY HEAT EXCHANGERS The purpose of cooling water heat exchangers is to reject waste heat from the plant equipment to the circulating water system. The pressure drop across the closed-cycle cooling water heat exchanger, including the filtration equipment and piping, shall equal the pressure drop across the condenser for balancing purposes. A plate heat exchanger is provided with a strainer upstream of the circulating water inlet. Strainer mesh size shall be coordinated with the heat exchanger manufacturer. Heat exchanger plate material shall be the same as the condenser tube material. HYDRAULIC GRADIENT A hydraulic gradient calculation should be performed for all once-through systems. A hydraulic gradient calculation allows quick determination of where protective devices may need to be located on a circulating water system. All hydraulic gradient calculations for multiple pump systems shall include the hydraulic gradient associated with the minimum flow, design flow and maximum flow conditions at the minimum static head condition. The hydraulic gradient calculation should also be performed for the no-flow condition. 14

15 SIPHON RECOVERY The seal weir elevation is set such that the absolute pressure head will be at least 1.52 meters of water greater than the fluid vapor pressure at all points in the system for any expected steady-state condition, including the no-flow system at rest case. Because the downstream dynamic loss is typically larger than the velocity head at the high point in the system, the most constraining steady-state case for siphon recovery will usually be the no-flow case. The maximum allowable elevation difference between the system high point (typically the top of the condenser water box) and the top of the weir should be calculated based on the siphon recovery. Seal weir design should consist of a sharp crested weir made of adjustable stop logs. Seal weirs that have adjustable weir elevations shall have instrumentation that alarms the operator when the system vacuum approaches the maximum allowable vacuum. Vapor pressure shall be calculated based on the operative fluid at the maximum fluid temperature that the system is expected to experience during steady-state operation. VACUUM PRIMING SYSTEM Air should be eliminated from the circulating water system prior to starting the circulating water pumps and should remain out of the system for proper operation. For most recirculating systems, this can be accomplished during system startup by simply bleeding the air out of the system through vents when filling the system. However, once-through systems operate with a siphon, which means that there is negative gauge pressure at the high points in the system. Therefore, a vacuum priming system is required to remove the air from the system. Circulating water systems that use a siphon tend to accumulate air at the condenser and auxiliary cooling water heat exchanger outlet water boxes during operation. This is because of the decrease in solubility of air in water associated with the pressure drop and temperature rise in the system. The purpose of the vacuum priming system is to remove air from the high points in the circulating water system during initial commissioning, as well as during the operation of the plant. 15

16 LIQUID TRANSIENT ANALYSIS The large flow rates of a typical circulating water system with flow velocities ranging from 1.8 to 3.6 m/s introduce the potential for significant pressure transients that could be damaging. Liquid transient analyses are performed on circulating water systems to prevent costs associated with damage to system components from water hammer and other unsteady flow phenomena. The potential risk of destruction of the large capital investment of a circulating water system and the threat of forced shutdown of the power plant are compelling reasons to perform such an analysis. A conservative selection of design pressure for all circulating water system components should provide a sufficient margin to protect against damage from water hammer. However, these design margins are not always sufficient and each system should be reviewed for water hammer potential. The modeling techniques available are accurate and the results are often used in the preparation of system operating procedures as well as the selection of surge protection devices, if needed. The typical analysis reviews normal valve and pump operating scenarios and foreseeable abnormal operating scenarios such as tripping of one or more large circulating water pumps or closure or opening of a valve within a circulating water system. To avoid a water hammer event, it is necessary to prevent the formation of a vapor cavity. This is accomplished by installing a vacuum breaking system. The purpose of a vacuum breaking system is to prevent the pressure in the system from dropping below vapor pressure. This is typically accomplished by admitting air into the system through an air valve or solenoid valve. The need for a vacuum breaking system is determined while performing a liquid transient analysis, as well as defining the system components. Once the general layout of the circulating water system is established and the pump capacities and pipe sizes are known, a study of water hammer potential should be performed. INITIAL FILLING AND PUMP LOGIC The circulating water pumps should not be used to fill the circulating water system. The entire circulating water system must be filled and be confirmed as full prior to starting the circulating water pumps. The use of the circulating water pumps to fill the circulating water system can lead to large transient pressures as the system becomes full of water. Water hammer pressure experienced during the filling of the circulating water system is directly proportional to the 16

17 circulating water flow rate. Circulating water pump flow rates cannot be throttled by the use of a pump discharge valve to effectively fill the circulating water system. Although the water hammer pressure can be reduced by controlling the fill rate using the circulating water pump discharge valve, the required valve position leads to excessive cavitation across the valve, and the circulating water pump is typically well below its rated minimum flow rate. These two conditions can lead to severe damage of the circulating water pump and its associated pump discharge valve. Once-through cooling systems should fill with a screen wash pump or an auxiliary cooling water pump. If the system is located on a tidally influenced water source, it is possible to reduce the time that is required to fill the system by filling during periods of high tide. This allows some system piping to be partially filled with water before other means of filling are started. A vacuum priming system should be used to complete the filling procedure. Condenser water boxes should be provided with level switches to ensure that water boxes are filled before starting the circulating water pumps. Certain pump logic must be used to ensure that the circulating water system components are protected during system transients. Typically, water hammer transient analysis provides detailed operating recommendations to prevent a water hammer event. The important cases that should be considered in pump logic include unscheduled circulating water pump trip, controlled circulating water pump start and shutdown. The pump start or shutdown is closely coordinated with its associated discharge valve operation to prevent water hammer events. All circulating water pumps shall be equipped with automatic air release valves located directly upstream of the circulating water pump discharge butterfly valve or located on the pump column above the discharge line. The air valve should be sufficiently sized to allow any air located in the pump column and piping upstream of the valve to be evacuated during a pump start without exceeding piping maximum design pressure. The valve should also allow for the air to be evacuated in a reasonable amount of time, so that the circulating water pump does not operate near its dead-head (no-flow) condition for an extended period of time. If the pump operates at its dead-head condition for an extended amount of time, the pump may experience excessive vibration. 17

18 SPACE CONSIDERATIONS/ACCESSIBILITY The available space for the pump intake structure and the degree of access required by the owner must be considered. Vertical wet pit pumps require the least amount of space; however, the rotating elements must be pulled out of the pit for bearing or impeller access. Vertical pumps with a pull-out design do allow easier maintenance, but at an increased capital cost. Typically, a double girder bridge crane is provided for maintenance of circulating water pumps, screens and stop logs. CONCLUSIONS Once-through circulating water system design provides the most efficient cycle heat rejection with the lowest capital and operating costs. However, it poses a number of challenges that need to be addressed during the design process. Selection and sizing of each component of a once-through circulating water system need special considerations. Studies such as a thermal plume model study, sump model study and liquid transient analysis are key to the safe design of a circulating water system. Selection of appropriate material and coating system based on water quality is important. Other special design considerations specific to a once-through circulating water system include use of siphon recovery to reduce circulating water pump head, use of hydraulic gradient plot to determine need of protective devices, use of vacuum priming systems to establish system prime during startup and operation, stop log-type seal weir construction to help in commissioning, and development of detailed procedure to fill, start and stop the circulating water system. All these factors need sufficient attention during once-through circulating water system design. REFERENCES 1. Lawrence F. Drbal, et. al., Power Plant Engineering Black & Veatch, Springer, Black & Veatch Holding Company All Rights Reserved. The Black & Veatch name and logo are registered trademarks of Black & Veatch Holding Company. 18